This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The purpose of this study is to determine whether the. radiotracer tailed (C]CNS5161 can be used to measure levels of N-methyl-D-aspartate (NMDA) receptors in the normal human brain and to see if these measures give us similar results each tithe. Tice ability to measure the level of NMDA receptors in the human brain will be important in understanding their role" in Alzheimer's Disease (AD). NMDA is a neurotransmitter in the brain that is linked with learning and memory, To image these NMDA receptors;we will use Positron Emission Tomography (PET) and a radiotracer balled (1C]CN55161. This radiotracer was developed to bind (stick) to NMDA receptors. PET is a special camera that takes pictures of the movement of a radiotracer in the body. This radiotracer is a radioactive chemical that is injected into the bloodstream in very small amounts and serves to indicate levels of NMDA receptors. You will receive three Positron Emission Tomography (PET) scans with the same radiotracer. Two of the scans will be done to image your brain. If you choose to go on memantirie therapy under a different protocol at New York University ((NYU), you will return for a third brain scan which is described on the next page in the boxed section.